Are introduced birds unimportant mutualists? A case study of frugivory in European blackbirds (Turdus merula)

Many types of birds regularly consume fleshy fruits and, as seed dispersers, perform important mutualistic services for plants. Some frugivorous birds have recently been introduced to geographic locales beyond their native range. Are non-native birds important frugivores in their introduced range? To answer this question, I observed native and introduced birds foraging for fruits in a New Zealand forest at approximately 5-day intervals for 5 years. I then compared fruit consumption patterns of European blackbirds (Turdus merula) to native New Zealand birds to determine whether blackbirds are important frugivores in New Zealand. I also compared the fruit diets of blackbirds to previously published dietary records from similar latitude forests in the United Kingdom. Results showed that blackbirds were among the most common frugivores at both New Zealand sites. They exhibited similar levels of dietary diversity to native bird species and consumed an unusually broad composition of fruit species. Introduced blackbird populations also exhibited remarkably similar patterns in dietary diversity and composition to populations in their native range. Results suggest that introduced birds can form important mutualistic partnerships with native plants, as seed dispersers, in the absence of contemporary coevolution.